This invention relates generally to a processor, or high temperature vacuum furnace, preferably for heating dental reconstruction products using sintered powder metal at very high temperatures, and relates more particularly to a special muffle chamber for heating dental reconstruction products in a vacuum environment.
The type of muffle chamber described above includes an elongated muffle tube in which the product to be fired is placed. The interior of the muffle tube defines the vacuum chamber and is first evacuated of atmospheric air so that a high degree of vacuum is achieved. Then the tube is rapidly heated preferably by electrical heating elements until an interior temperature of about 1200.degree. C. (2192.degree. F.) by radiant thermal energy is achieved. The energy passes from the heating elements first to the cylindric wall of the muffle tube and thereupon from the heated tube wall to the product to be fired at the general center of the vacuum chamber formed by the tube.
The primary problem encountered on muffle tubes now in use is that the length of the tube is determined in accordance with thermal shock resistance characteristics of the tubes, its thermal conductivity, and in accordance with the maximum use temperature of silicone rubber seals at the longitudinal ends of the tube that are necessary to maintain the vacuum in the tubes.
Many ceramic materials are unable to withstand sudden changes in temperature without flaking, dunting, spalling, cracking, or other form of disintegration. The extent to which a material can withstand different temperatures along its length without such disintegration or cracking can be referred to as thermal shock resistance, which is often defined in terms of the maximum temperature interval through which the material can be rapidly chilled without fracturing or otherwise disintegrating. A discussion on this subject can be found in The Chemistry and Physics of Clays and Other Ceramic Materials by R. W. Grimshaw, Fourth Edition, Wiley-Interscience, New York, 1971, pages 949-955; and in Glass Ceramics by P. W. McMillan, Academic Press, New York, 1964, pages 191-193. The thermal shock resistance characteristic is such that a maximum thermal differential, or gradient, per unit length of the tube cannot be exceeded without disintegration of the tube. Because the maximum use temperature of the silicone rubber seals is about 200.degree. C. (392.degree. F.), the longitudinal ends of the tube must be kept below that temperature, and preferably below 150.degree. C. (302.degree. F.). Thus, sufficient length of tube is required to allow cooling from a heating chamber temperature of 1200.degree. C. at its longitudinal center to 150.degree. C. at its ends without exceeding the maximum thermal shock resistance. Therefore, if no insulation were utilized to control the cooling of the tube along its length, and only distance from the heating element allowed cooling, a thermal gradient, possibly only 30.degree. C. (86.degree.) per inch would be achieved. Accordingly, a tube length of about 6 feet, that is, with the ends at about 35 inches from the longitudinal center, would be the necessary result. The cumbersome aspects of such a dimension are apparent, and even more so when the mechanisms associated with the tube, such as electric heating elements, the vacuum apparatus, the outer blowers, and so on, are taken into consideration.
The invention allows for a shortened muffle tube to a more manageable length. The invention includes placing insulation around a shortened tube made of a ceramic material which has a relatively low thermal shock resistance characteristic compared to other materials, such as metals, and the like, but the novel structure of the furnace, maximizes the thermal gradient to come close to the maximum thermal shock resistance of the tube material. Accordingly, the shortest tube length is achieved. Insulation wrapped around the tube placed above and below the center area of the longitudinal dimension of the tube, that is, above and below the heating elements, have resulted in the cracking of the walls of the tube at the point where the insulation began at the walls of the tube in the plane perpendicular to the center axis of the tube upon heating of the tube. This result can be attributed to a too sudden reduction of temperature at the point of juncture between the hot heating chamber and the plane at the insulation wrapping.